Low Dissolved Oxygen Concentrations Research Articles

Patterns and Drivers of CO2 and CH4 Fluxes in an Urbanized River Network and Their Response to Restoration

AbstractCarbon evasion from urban river networks becomes increasingly significant as urbanization accelerates. However, there remains a limited understanding of the overall carbon emission impact integrating CO2 and CH4 dynamics, particularly in response to ecological restoration efforts. In this study, we investigated patterns of fluvial CO2 and CH4 diffusive fluxes across an urban river network in Wuxi, China. Our results reveal that water quality variables, especially dissolved oxygen (DO) and phosphorus content, predominantly influence the variability of carbon emissions. These factors exhibit a stronger correlation with CO2 emissions compared to CH4, indicating a net increase in carbon emissions as water quality deteriorates. Seasonally, higher water temperatures, phosphate levels, and lower DO concentrations lead to increased carbon emissions during summer months. Spatially, areas with lower carbon emissions (averaged 86 mmol m−2 d−1 CO2 and 0.13 mmol m−2 d−1 CH4) are primarily situated near the lake and in river sections where significant water quality improvements have been achieved through ecological restoration efforts. Cluster analysis shows that over 60% of high‐carbon emission (averaged 162 mmol m−2 d−1 CO2 and 1.21 mmol m−2 d−1 CH4) sites in the study area have undergone ecological restoration, suggesting potential for further carbon emission reduction through enhanced restoration practices. Our findings underscore the importance of implementing carbon reduction strategies such as nutrient removal and aeration for oxygenation within water ecological restoration initiatives. Effective matching of restoration strategies holds further potential for mitigating carbon emissions from urban river networks.

Hypoxia extreme events in a changing climate: Machine learning methods and deterministic simulations for future scenarios development in the Venice Lagoon

Climate change pressures include the dissolved oxygen decline that in lagoon ecosystems can lead to hypoxia, i.e. low dissolved oxygen concentrations, which have consequences to ecosystem functioning including biogeochemical cycling from mild to severe disruption. The study investigates the potential of machine learning (ML) and deterministic models to predict future hypoxia events. Employing ML models, e.g. Random Forest and AdaBoost, past hypoxia events (2008–2019) in the Venice Lagoon were classified with an F1 score of around 0.83, based on water quality, meteorological, and spatio-temporal factors. Future scenarios (2050, 2100) were estimated by integrating hydrodynamic-biogeochemical and climate projections. Results suggest hypoxia events will increase from 3.5 % to 8.8 % by 2100, particularly in landward lagoon areas. Summer prediction foresee a rise from 118 events to 265 by 2100, with a longer hypoxia-prone season. This model is a valuable tool for developing hypoxia scenarios, aiding in identifying restoration hotspots for climate-threatened lagoons.

Molecular response and adaptation mechanism of Microcystis aeruginosa under metalimnetic oxygen minimum conditions

Reservoirs are important drinking water sources. The metalimnetic oxygen minimum (MOM) usually occurs periodically in summer and autumn in deep-water reservoirs due to algae blooms and thermal stratification. This study aimed to explore the physiological and molecular responses of Microcystis aeruginosa (M. aeruginosa) under MOM conditions (darkness coupled with low dissolved oxygen (DO) concentration, hydrostatic pressure, and nutrient starvation). The comprehensive response of M. aeruginosa suggested that MOM conditions led to an immediate collapse of gas vesicles. This was followed by a gradual inhibition of photosynthesis by disturbing the electron transport chain and a significant downregulation of energy metabolism and carbohydrate metabolism. The active cells were approximately 5 % and > 45 % under MOM aerobic (3.0–7.0 mg/L DO) and anaerobic conditions (< 0.5 mg/L DO), respectively, for 20 days. In addition, a single exposure to darkness or pressure accelerated the decay of M. aeruginosa cells; however, MOM conditions with a low DO concentration had the opposite effect. The survival of M. aeruginosa cells under MOM conditions could be attributed to stringent response and the activation of HIF-1 signal when DO concentration decreased to < 2.0 mg/L by promoting the formation of cellular quiescence and resource redistribution. This study sheds light on the molecular response and adaptation mechanism of M. aeruginosa under MOM conditions.

Enhancing corrosion resistance of T91 F/M steel in liquid lead-bismuth (LBE) by slurry FeAl coating

A FeAl coating was fabricated on T91 steel via slurry aluminizing. The corrosion behavior of coated and uncoated samples was assessed in static LBE (lead-bismuth eutectic) with two both high and low dissolved oxygen concentrations at 550 °C. The coating was mostly intact and exhibited great corrosion resistance compared to the uncoated specimens regardless of the oxygen concentration. That is because the coating can form a protective alumina film on the surface at extremely low dissolved oxygen level. This coating is of significant engineering value in enhancing the corrosion resistance of Fe base alloy in LBE.

Comparative studies of the long-term corrosion behavior of Zr-Sn-Fe-Cr-Ni alloys in pure water at 360°C/18.6 MPa with high and low dissolved oxygen content

This study compared the uniform corrosion behavior of Zr-Sn-Fe-Cr-Ni alloys in deionized water at 360°C/18.6 MPa with high and low concentration of dissolved oxygen (DO). It was found that high concentration of DO accelerated the corrosion rate of the Zr-Sn-Fe-Cr-Ni alloys and led to an earlier corrosion transition. Increased DO concentration resulted in a higher content of t-ZrO2 near the metal-oxide interface, which induced greater in-plane compressive stress in the oxide film and a high-level phase transformation from t-ZrO2 to m-ZrO2. This, in turn, led to an earlier occurrence of corrosion transition.

Effect of dissolved oxygen on sulfur autotrophic denitrification and how to address it: An experimental and modelling work

Sulfur autotrophic denitrification (SAD) using elemental sulfur as the electron donor has aroused increasing interest of its application in treating secondary effluent from wastewater treatment plants (WWTPs). However, high influent dissolved oxygen (DO) in secondary effluent would limit the SAD process. This study examined the effect of different DO concentrations on SAD. Results revealed that both low (0–0.5 mg/L) and moderate (2.5–3.5 mg/L) DO concentrations would not harm the nitrate removal rate (NRR) (p > 0.05). However, high DO concentration (5.5–6.5 mg/L) significantly decreased the NRR (p < 0.05) through strong competition over the nitrate for electrons and cutting the relative abundance of sulfur-oxidizing bacteria (SOB). Both modeling and experimental results showed that applying internal reflux could serve as a strategy to mitigate the negative effect of high DO concentration, while keeping an appropriate ratio was crucial. When treating real membrane bioreactor (MBR) effluent with high DO concentration (5.5–6.5 mg/L), an internal reflux ratio of 0.5 boosted the NRR by 1.5 times. This study provided potential reference and strategy for dealing with high DO concentration wastewater by applying SAD technology.

A novel process based on marine heterotrophic nitrification and aerobic denitrification bacteria for treating mariculture wastewater: Performance, biofilm characteristics and microbiome responses to dissolved oxygen

A novel reactor utilizing marine heterotrophic nitrification and aerobic denitrification (MHNAD) bacteria offers an effective approach for treating mariculture wastewater. However, the impact of dissolved oxygen (DO) concentration on its performance has been understudied. This research investigated how varying DO concentrations influence the performance, biofilm characteristics, and microbiome of the reactor when inoculated with MHNAD bacteria. The results demonstrated that reducing the DO concentration from 6 mg/L to 2 mg/L decreased the removal efficiency of chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) but significantly improved nitrate nitrogen (NO3−-N) removal. Higher DO concentrations (6 mg/L) promoted the production of extracellular polymeric substances (EPS), particularly proteins. Microbial analysis revealed that MHNAD bacteria, particularly the genus Zobellella, consistently dominated the bioreactor, though their abundance decreased at lower DO concentrations. Bugbase analysis indicated that biofilm formation and aerobic bacterial abundance decreased as DO concentration declined. Additionally, metabolic pathway analysis suggested that lower DO levels might inhibit glycolysis and TCA cycle processes, while also altering the primary nitrogen removal pathways.

The Occurrences of Harmful Algal Blooms (HABs) Species and Trophic Status Update in Kedung Ombo Reservoir

Graphical Abstract Highlight Research Water quality conditions in Kedung Ombo Reservoir. Phytoplankton species and their abundance in Kedung Ombo. Potential existence and dominance of HABs species. The updated trophic status is mapped based on the TSI, Secchi depth, chlorophyll-a, and total phosphorus. Abstract Anthropogenic inputs affect the quality of freshwater ecosystems which causes ecological and health problems to aquatic ecosystems. Harmful algal blooms (HABs) associated with cyanotoxins often occur in nutrient-rich or eutrophic freshwater ecosystems. Kedung Ombo Reservoir in Indonesia has been previously classified as eutrophic to hypertrophic. Therefore, this study aimed to identify the occurrences of potential HABs species, measure the bio-physico-chemical water quality parameters, and update the trophic status of Kedung Ombo Reservoir. Sampling was done thrice during the dry season in 2022 from 5 stations. Twenty-two species of phytoplankton were observed in Kedung Ombo Reservoir. Anabaenopsis sp., Aphanizomenon sp., Ceratium sp., Mougeotia sp., Pandorina sp., and Ulothrix sp. were identified as potentially harmful species. Among those, the potentially HABs species, Aphanizomenon sp. was the most abundant (179,344 cells/L) and Cyanophyceae (205,539 cells/L) was the dominant group of phytoplankton. Kedung Ombo Reservoir had a water temperature of 29.49±0.41°C, phosphate of 0.27±0.25 mg/L, and alkaline pH of 7.90±0.39. Kedung Ombo Reservoir also had low transparency coupled with low dissolved oxygen concentration. The occurrences of HABs species were correlated with transparency and dissolved inorganic nutrients, especially phosphate concentrations. Kedung Ombo Reservoir showed eutrophic conditions based on Secchi depth, chlorophyll-a, total phosphorus, and TSI. Based on research findings, control and mitigation efforts are needed to overcome the eutrophication problems which disrupt the balance of the aquatic ecosystem in the Kedung Ombo Reservoir.

Assessment of temporal and spatial variations of water quality parameters in the Zarafshan River basin

AbstractRiver ecosystems in Central Asia face significant stress from environmental changes and pollution. This study assesses temporal and spatial variations in water quality parameters within the Zarafshan River Basin using retrospective data and field measurements. Water quality indicators, including electrical conductivity (EC), total suspended solids (TSS), ammonium nitrogen (N‐NH4), nitrite nitrogen (N‐NO2), nitrate nitrogen (N‐NO3), temperature (T), chemical oxygen demand (COD), dissolved oxygen (DO), and discharge, were analyzed using the Pearson's correlation coefficient and ANOVA, with the Mann–Kendall (MK) test detecting trends over time. Obtained results indicate significant seasonal effects with elevated TSS during summer, increasing sediment load and changing aquatic habitats. The strong inverse correlation (–0.89) between DO and N‐NH4 signifies ecological challenges particularly in low DO concentrations during summer (3.25 mg L–1). Long‐term analysis identifies Navoiazot chemical factory as a major pollution hotspot. Spatial analyses based on extended sampling have revealed the Siab and Dargom canals and Samarkand City as major pollution sources of elevated N‐NO2 and COD, respectively. Trends at various gauging stations (MK‐test) show increasing EC (τ = 0.72) and N‐NH4 (τ = 0.46) levels, with decreasing TSS, N‐NO3, T, and COD levels over time. Recommendations include targeted measures to reduce pollution at the Navoiazot factory and downstream, introducing sustainable agriculture practices, increasing public awareness for environmental conservation, and improving urban wastewater treatment to meet water quality requirements for different users.

Metatranscriptomic Analysis Reveals Synergistic Activities of Comammox and Anammox Bacteria in Full-Scale Attached Growth Nitrogen Removal System.

Leveraging comammox Nitrospira and anammox bacteria for shortcut nitrogen removal can drastically lower the carbon footprint of wastewater treatment facilities by decreasing aeration energy, carbon, alkalinity, and tank volume requirements while also potentially reducing nitrous oxide emissions. However, their co-occurrence as dominant nitrifying bacteria is rarely reported in full-scale wastewater treatment. As a result, there is a poor understanding of how operational parameters, in particular, dissolved oxygen, impact their activity and synergistic behavior. Here, we report the impact of dissolved oxygen concentration (DO = 2, 4, 6 mg/L) on the microbial community's transcriptomic expression in a full-scale integrated fixed film activated sludge (IFAS) municipal wastewater treatment facility where nitrogen removal is predominantly performed by comammox Nitrospira and anammox bacterial populations. 16S rRNA transcript compositions revealed anammox bacteria and Nitrospira were significantly more active in IFAS biofilms compared to suspended sludge biomass. In IFAS biofilms, anammox bacteria significantly increased hzo expression at lower dissolved oxygen concentrations and this increase was highly correlated with the amoA expression levels of comammox bacteria. Interestingly, the genes involved in nitrite oxidation by comammox bacteria were significantly more upregulated, relative to the genes involved in ammonia oxidation with decreasing dissolved oxygen concentrations. Ultimately, our findings suggest that comammox Nitrospira supplies anammox bacteria with nitrite via ammonia oxidation and that this synergistic behavior is dependent on dissolved oxygen concentrations.

New insights on the efficiency and mechanism of dissolved oxygen regulating the redox removal of organic pollutants by S(IV) system

Recently, sulfite (S(IV)) has been extensively studied in the field of wastewater treatment. Multiple investigations have indicated that dissolved oxygen (DO) plays a crucial role in the S(IV) system. However, the mechanism of the effect of DO on the S(IV) system further investigation is required. Herein, the oxidative and reductive removal performance and mechanisms are modulated by adjusting the DO concentration in the S(IV) system. Specifically, an increase in DO concentration promotes the formation of HO• and SO4•−, while SO3•− is more prominently enriched at lower DO concentrations. Within a specific range, as the DO concentration increased, the oxidation proportion increased, while the reduction proportion decreased. In addition, under aerobic conditions, oxidation and reduction were synchronized to remove p-nitrophenol (PNP) and most of the organic matter oxidized into small molecules by HO• and SO4•−. In the absence of oxygen, only PNP was reduced to p-aminophenol (PAP) by ZVI and SO3•−. Surprisingly, the same phenomenon was observed for chlorinated organic pollutants. A comprehensive understanding of the mechanisms through which DO concentration modulates the specific removal of characteristic organic pollutants by oxidation and reduction is essential for the S(IV) system in water treatment processes.

Isotopomer labeling and oxygen dependence of hybrid nitrous oxide production

Abstract. Nitrous oxide (N2O) is a potent greenhouse gas and ozone depletion agent, with a significant natural source from marine oxygen-deficient zones (ODZs). Open questions remain, however, about the microbial processes responsible for this N2O production, especially hybrid N2O production when ammonia-oxidizing archaea are present. Using 15N-labeled tracer incubations, we measured the rates of N2O production from ammonium (NH4+), nitrite (NO2-), and nitrate (NO3-) in the eastern tropical North Pacific ODZ and the isotopic labeling of the central (α) and terminal (β) nitrogen (N) atoms of the N2O molecule. We observed production of both doubly and singly labeled N2O from each tracer, with the highest rates of labeled N2O production at the same depths as the near-surface N2O concentration maximum. At most stations and depths, the production of 45N2Oα and 45N2Oβ were statistically indistinguishable, but at a few depths there were significant differences in the labeling of the two nitrogen atoms in the N2O molecule. Implementing the rates of labeled N2O production in a time-dependent numerical model, we found that N2O production from NO3- dominated at most stations and depths, with rates as high as 1600 ± 200 pM N2O d−1. Hybrid N2O production, one of the mechanisms by which ammonia-oxidizing archaea produce N2O, had rates as high as 230 ± 80 pM N2O d−1 that peaked in both the near-surface and deep N2O concentration maxima. Based on the equal production of 45N2Oα and 45N2Oβ in the majority of our experiments, we infer that hybrid N2O production likely has a consistent site preference, despite drawing from two distinct substrate pools. We also found that the rates and yields of hybrid N2O production were enhanced at low dissolved oxygen concentrations ([O2]), with hybrid N2O yields as high as 20 % at depths where [O2] was below detection (880 nM) but nitrification was still active. Finally, we identified a few incubations with [O2] up to 20 µM where N2O production from NO3- was still active. A relatively high O2 tolerance for N2O production via denitrification has implications for the feedbacks between marine deoxygenation and greenhouse gas cycling.

Development of an integrated fixed-film activated sludge (IFAS) reactor treating landfill leachate for the biological nitrogen removal through nitritation-denitritation

The current work investigated the performance of an Integrated Fixed-Film Activated Sludge Sequencing Batch Reactor (IFAS-SBR) for Biological Nitrogen Removal (BNR) from mature landfill leachate through the nitritation-denitritation process. During the experimental period two IFAS-SBR configurations were examined using two different biocarrier types with the same filling ratio (50%). The dissolved oxygen (DO) concentration ranged between 2 and 3 mg/L and 4–6 mg/L in the first (baseline-IFAS) and the second (S8-IFAS) setup, respectively. Baseline-IFAS operated for 542 days and demonstrated a high and stable BNR performance maintaining a removal efficiency above 90% under a Nitrogen Loading Rate (NLR) up to 0.45 kg N/m3-d, while S8-IFAS, which operated for 230 days, was characterized by a limited and unstable BNR performance being unable to operate sufficiently under an NLR higher than 0.20 kg N/m3-d. It also experienced a severe inhibition period, when the BNR process was fully deteriorated. Moreover, S8-IFAS suffered from extensive biocarrier stagnant zones and a particularly poor sludge settleability. The attached biomass cultivated in both IFAS configurations had a negligible content of nitrifying bacteria, probably attributed to the insufficient DO diffusion through the biofilm, caused by the low DO concentration in the liquid in the baseline case and the extensive stagnant zones in the S8-IFAS case. As a result of the high biocarrier filling ratio, the S8-IFAS was unstable and low. This was probably attributed to the mass transfer limitations caused by the biocarrier stagnant zones, which hinder substrate and oxygen diffusion, thus reducing the biomass activity and increasing its vulnerability to inhibitory and toxic factors. Hence, the biocarrier filling fraction is a crucial parameter for the efficient operation of the IFAS-SBR and should be carefully selected taking into consideration both the media type and the overall reactor configuration.

Food web collapse and regime shift following goldfish introduction in permanent ponds.

In a global context of invasive alien species (IAS), native predators are often eradicated by functionally different IAS, which may induce complex cascading consequences on ecosystem functioning because of the key role predators play in structuring communities and stabilizing food webs. In permanent ponds, the most abundant freshwater systems on Earth, global human-mediated introductions of alien omnivores such as the pet trade goldfish are driving broad-scale patterns of native predators' exclusion, but cascading consequences on food web structure and functioning are critically understudied. We compared food webs of naturally fishless ponds versus ponds where dominant native predators (newts) had been extirpated by invasive goldfish within the last decade. Integrating community-wide isotopic, taxonomic and functional traits approaches, our study reveals that pond food webs collapsed in both vertical and horizontal dimensions following goldfish introduction and the associated exclusion of native predators. Consumer taxonomic diversity was drastically reduced, essentially deprived of amphibians as well as predatory and mobile macroinvertebrates to the profit of burrowing, lower trophic level consumers (detritivores). Changes in community structure and function underlined a regime shift from a macrophyte-dominated system mainly characterized by benthic primary production (periphyton), to a macrophyte-depleted state of ponds hosting communities mainly associated with phytoplankton primary production and detritus accumulation, with higher tolerance to eutrophication and low dissolved oxygen concentration. Results underline major impacts of widely introduced omnivores such as the goldfish on the functioning of pond ecosystems with potentially dramatic consequences on the key ecosystem services they deliver, such as global biodiversity support or water quality improvement. They also shed light on the key role of submerged aquatic vegetation in supporting diverse communities and complex food webs in shallow lentic systems and call for urgent consideration of threats posed by IAS on ponds' ecosystems by managers and policymakers.

Effects of oxygen concentration on the corrosion behavior of high entropy alloy AlCoCrFeNi in simulated deep sea

In light of the low dissolved oxygen concentration in the deep sea, the corrosion mechanisms of the high entropy alloy (HEA) AlCoCrFeNi in artificial seawater with varying oxygen concentrations (2.0, 4.0, 7.0 mg/L) were studied. As the oxygen concentration decreases, the alloy's free corrosion potential decreases, and at 2.0 mg/L, the corrosion rate is 421 times higher than that at 7.0 mg/L. The corrosion form transforms from pitting to uniform corrosion. The primary reasons for this are the passivation film is thin under low oxygen concentration conditions, as well as the preferential dissolution of the alloy elements Al and Ni due to their high activity and “local acidizing” properties, respectively. In designing a super corrosion-resistant high entropy alloy for use in the deep sea, it is advisable to avoid the use of element Al and to add Ni with caution.

Treatment of Wastewaters with Different Ammonium Concentrations in a Sequencing Batch Reactor by Nitritation and Denitritation Processes

In the treatment of industrial and municipal wastewaters, biological nitrification and denitrification processes have been widely applied methods. However, in recent years, experimental studies have been carried out on nitritation and denitritation processes in order to reduce the cost of wastewater treatment. In this study, the removal of synthetic wastewaters containing ammonium at various concentrations through nitritation and denitritation processes was investigated in a sequence batch reactor (SBR). The SBR was operated at a low dissolved oxygen concentration of 0.7±0.2 mg/L, while the initial pH level and temperature were kept constant at 7.5 and 25 ℃, respectively, which were below the defined optimal level for nitritation. For the initial concentrations of 100, 150, and 200 mg NH4-N/L, 91, 142, and 180 mg NOx-N were produced, respectively with the specific nitritation rates of 0.69, 0.95, and 0.81 mg NH4-N/mg MLVSS.hour. Considering the operating conditions of nitritation, approximately NO2-N/NOx-N ratio of 80% could be considered as successful with the combined effect of DO limitation and NH3 inhibition on the nitrite oxidizing bacteria. During the first days of nitritation and denitritation processes, change of nitrogen concentrations was negligible because the adaptation of organisms to the oxic and anoxic conditions. After completing nitritation stage, denitritation was carried out by using methanol as an electron donor for the initial concentrations of 91, 142, and 180 mg/L of NOx-N in 96, 120 and 264 hours of reaction times, respectively.

Dissolved oxygen dependence of Microbe-Derived dissolved organic nitrogen dynamics during membrane biofilm-based linear alkylbenzene sulfonate mineralization

Oxygen-based membrane biofilm reactor (O2-MBfR) can achieve bubble-free efficient linear alkylbenzene sulfonate (LAS) mineralization. However, effluent microbially derived dissolved organic nitrogen (mDON) reduces recycled water quality and induces water eutrophication. How dissolved oxygen (DO) impacts the dynamics of mDON during LAS biodegradation is unknown. Here, we explored the fate of mDON releasing, molecular characteristics and formation metabolisms in the O2-MBfR responding to DO dynamics using Fourier transform ion cyclotron resonance mass spectrometry and metagenomic analyses. Results showed that DO presents a negative impact (λ = -0.242, P < 0.001) on mDON production, but can positively influence mDON reduction through influencing total organic nitrogen (TDN) removal. DO of 0.3 mg/L led to the lowest concentration of mDON with the least number of organic molecular formulas and high proportion of stable N2– and N3-containing organics. Much lower (0.0 mg/L) or higher (0.6 mg/L) DO concentrations favored dissimilatory-based ammonia formation and assimilatory nitrogen fixation that enhancing mDON formation. Firmocutes were the dominant bacteria that contributing to mDON formation. TDN (λ = 0.844, P < 0.001) and pH (λ = 0.248, P < 0.01) directly affected the effluent mDON concentration. The relatively low effluent mDON under anoxic condition suggested that DO controlling can be an effective strategy to minimize bioavailable mDON discharges in the O2-MBfR, which showed great implication potential for high-quality reclaimed water production and eutrophication control.

Effect of low dissolved oxygen concentration on the tribocorrosion properties and ion release of Ti-13Nb-13Zr

Effect of low dissolved oxygen concentration on the tribocorrosion properties and ion release of Ti-13Nb-13Zr

Genesis of gray sandstone within the red beds in HLJ-DL uranium deposit, southwest Songliao Basin and its relationship with uranium mineralization

The search for uranium deposits in the red beds is currently a hotspot in China's sandstone-type uranium exploration efforts. The typical occurrence of uranium deposits is within gray sandstone layers amidst the red beds, where the distribution of these gray sand bodies dictates the presence of uranium deposits. Hence, delving into the genesis of gray sandstone within the red beds proves beneficial for both uranium exploration and unraveling the ore-forming mechanisms. However, there is currently significant controversy surrounding the genesis of gray sandstone in the red beds. This paper focuses on the typical uranium deposit in the red beds—the HLJ-DL uranium deposit in the southwestern part of the Songliao Basin. It investigates the genesis of gray sandstone within uranium-bearing rock series and its relationship with uranium mineralization. The lithology of representative drill cores, including the red sandstone for comparative study, is methodically examined, along with their elemental geochemistry as well as carbon and oxygen isotope characteristics. The petrological characteristics suggest that both the gray and red sandstone share similar compositions of clastic particles, indicating minimal influence of the source rock on the sandstone color. The gray sandstone displays comparable TOC content (averaging around 0.05 %) and δ18OPDB values (averaging around −16 ‰) to the red sandstone. However, it exhibits relatively higher δ13CPDB values (averaging −0.26 ‰), whereas the average δ13CPDB value for the red beds is −1.61 ‰. The data indicate that the gray sandstone is less affected by hydrothermal or hydrocarbon fluids. Compared to the red sandstone, which has relatively higher Cu/Al values (averaging 1.09) and Fe3+/Fe2+ ratios (averaging 3.37), the gray sandstone exhibits low Cu/Al values (averaging 0.44) and a low Fe3+/Fe2+ ratio (averaging 0.36), suggesting its formation in a relatively reducing environment. The reducing conditions necessary for the formation of gray sandstone may stem from the climate becoming relatively warm and humid, irrespective of whether it originates from the original sedimentary environment or is induced by the reducing pore water discharged from dark mudstones. The chemical weathering index of fine-grained sediments indicates that, the gray sandstones were commonly configured in relatively humid-warm climate with fairly low dissolved oxygen content in the bottom water. Additionally, the deposition of the uranium-bearing rock series experienced alternating climatic changes, rather than consistently being in a humid-warm climate. This resulted in the sand bodies with low TOC content, insufficient reducing potential, and uranium mineralization primarily occurring in the interior of the basin. Organic matter has a relatively weak control over uranium mineralization, whereas uranium mineralization shows a certain correlation with iron reduction. The ore-forming fluid was uranium-rich oxygenated atmospheric water. Dark-colored mudstones, formed under humid-warm climatic condition, contain relatively high TOC contents (avg. 0.43 %) and exhibit a strong reducing capacity. The reducing pore water in dark-colored mudstones excreted into the adjacent sandstones under compaction during the early diagenesis, is able to serve as a vital reducing agent in later-stage uranium mineralization. Additionally, certain dark-colored mudstones with high uranium content (up to 944 ppm) undergo significant uranium pre-enrichment, providing a partial uranium source for uranium mineralization.

Applying Harmony Degree Equation and TOPSIS Combined with Entropy Weights in Surface Water Classification

This study classified surface water quality in Can Tho city using the Eutrophication index, Harmony Degree Equation (HDE), and Technique of Order Preference by Similarity to Ideal Solution (TOPSIS). Water quality data were collected in two seasons at 38 locations with 18 parameters, including temperature, pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), nitrite (N-NO2-), nitrate (N-NO3-), ammonium (N-NH4+), orthophosphate (P-PO43-), Fe, F-, Pb, As, Hg, coliform, chlorine-, and phosphorus-based pesticides. Water quality parameters are compared with national technical regulations on surface water quality (QCVN 08-MT:2015/BTNMT). The HDE method based on entropy weight has been applied to evaluate the comprehensive harmony degree of water quality for various purposes. In addition, the TOPSIS was also used to rank water quality at each location and determine the priority level that required mitigation and treatment solutions. Surface water quality in the study area had low dissolved oxygen content and was contaminated with TSS and coliform in both seasons. Water quality in the rainy season tends to decrease compared to the dry season. Based on HDE results, water quality in the study area in the dry season was assessed as suitable for domestic activities (needs treatment), irrigation, and navigation (HDII = 0.922), while the rainy season was suitable for irrigation and navigation (HDIII= 1.00). Moreover, surface water in the study area was in a state of potential eutrophication (EI &gt; 0), in which eutrophication was higher during the dry season. The SW25 and SW28 were the most seriously eutrophic in the dry and rainy seasons, respectively. TOPSIS analysis indicated that SW22 and SW28 need treatment measures in both seasons; furthermore, SW2-SW4 (dry season) and SW23 (rainy season) also need appropriate management and impact mitigation solutions. SW4 was affected by the most significant seasonal impacts, which have high priority in the dry season and are lowest in the rainy season. Therefore, future studies are needed to identify specific sources of variation at these locations to reduce impacts. The study results provide helpful information for the decision-making process and water quality management. Doi: 10.28991/CEJ-2024-010-04-012 Full Text: PDF